The anaerobic conversion of complex organic matter to CH4 is an essential link in the global carbon cycle. In freshwater anaerobic environments, the organic matter is decomposed to CH4 and CO2 by a microbial food chain that terminates with methanogens that produce methane primarily by reduction of the methyl group of acetate and also reduction of CO2. The process also occurs in marine environments, particularly those receiving large loads of organic matter, such as coastal sediments. The great majority of research on methanogens has focused on marine and freshwater CO2-reducing species, and freshwater acetate-utilizing species. Recent molecular, biochemical, bioinformatic, proteomic, and microarray analyses of the marine isolate Methanosarcina acetivorans has revealed that the pathway for acetate conversion to methane differs significantly from that in freshwater methanogens. Similar experimental approaches have also revealed striking contrasts with freshwater species for the pathway of CO-dependent CO2 reduction to methane by M. acetivorans. The differences in both pathways reflect an adaptation by M. acetivorans to the marine environment.